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It's supposed to be a family-friendly game, but a global scavenger hunt group caught up in the case of missing Australian boy William Tyrrell has faced a series of brushes with the law.

On Sunday, a cryptic message painted on a tree trunk on the NSW mid-north coast led police to lock down the area and send in armed officers and helicopters as they investigated possible links to the abduction.

The pink graffiti reading "Jesus saves William Tyrrell" had been scrawled just above the hiding place of a mysterious glass jar filled with assorted items.

The jar was part of a real-world treasure hunt called "geocaching", which involves following GPS coordinates posted online to find hidden "caches" - containers containing small trinkets.

Since the game's creation in 2000, more than a hundred of the boxes have been mistaken for security threats, and X-rayed, defused, shot at and blown up. The suspicious-looking packages have caused lockdowns around the world, and the behaviour of players hunting for the treasure has frequently provoked alarm among so-called "Muggles" unacquainted with the game.

A cryptic message was spray-painted on a trunk above where the "geocache" was hidden. Photo / News Corp Australia

A cryptic message was spray-painted on a trunk above where the "geocache" was hidden. Photo / News Corp Australia

There are now six million geocachers worldwide. Last August, a bomb squad and robot were called after a player planted a pill bottle wrapped in camouflage tape in a Denver park.In 2013, Oregon police threatened to charge and fine a "ridiculous" geocacher who planted a section of dark-painted PVC pipe, capped at both ends, in a popular wetlands area. The walk was shut down, emergency services put on standby and a bomb squad called in.Just two days before, there had been a similar drama over a "geo bomb" on a seaside trail in Northern Ireland.

In 2011, the army and a bomb squad were called in after customers at a cafe in northern England spotted a geocacher leaving a package under a flowerpot.

One geocacher wrote on the game's official website that they had been arrested and fined $10,000 for picking up a cache at Los Angeles International airport, while a Pennsylvania neighbour called the police after a container was left in a tree by their home.

Scott Berks, a graphic designer from Illinois, told the Wall Street Journal two police officers had drawn their guns on him while he was geocaching at night along the Chicago River.

Geocaching websites issue advice on how to avoid clashing with the authorities, for example, by using clear plastic containers, notifying park rangers and avoiding sensitive locations. Player Andrew Smith, from North Carolina, suggests wearing a construction workers' vest or looking at your mobile phone to avoid arousing suspicion when searching.

The awkward encounters have even inspired T-shirts reading, "Don't Call the Police - We're Only Geocaching".

But players have on occasion helped the police solve mysteries, with one family finding the body of a man who took his own life on a Minnesota trail months after he vanished.

James Finger, president of Geocachers NSW, told news.com.au: "It's pure coincidence that trunk that was graffitied happened to have a geocache from four years ago hidden beneath."I can't say whether whoever did it found the geocache, but if they did they didn't log it.

"There are two million hidden around the world. Bomb scares are one of the hardest things to contain, people see them and don't know what they are.

"It's not mainstream but it's getting more so, it's part of the recreational bushwalking landscape."

He said the community "had to grit its teeth" about some of the negative feedback, with members of the public associating geocaching with paedophilia and saying they had an agenda after the Tyrrell graffiti appeared this weekend.

"Geocaching is a family-friendly hobby that appeals to people of all ages and lifestyles, including kids, who love finding the treasure that is hidden in a lot of them," he said.

NSW Police said in a statement on Monday: "About 6.30pm yesterday, a member of the public contacted police after a reference to William Tyrrell was found painted on a tree in a state forest near Stewarts River, about 10km south of Kendall.

"A glass jar with a number of items was located beneath the tree. Local police attended and informed detectives attached to Strike Force Rosann. Police secured the area and seized the items.

"Following inquiries, police have determined the items and painting were a result of 'geocaching', an outdoor activity in which participants use GPS coordinates to hide and seek items.

"Given the circumstances of William's disappearance, police are disappointed that it has been included in a recreational game. Police maintain their appeal for anyone with information concerning William's disappearance to contact Crime Stoppers."